1. Field of the Invention
This invention relates to an optical disk medium which records and reproduces information using light, and in particular to a super-resolution optical disk medium which reads out recording marks at equal to or less than an optical resolution determined by a diffraction limit light spot diameter.
2. Description of the Related Art
Due to recent advances in information technology, information communications and multimedia technology, there is an increasing demand for higher densities and higher capacities of optical disk media. The upper limit of recording density of an optical disk medium is mainly limited by the beam diameter of a light spot which records or reads out information. It is known that, if the wavelength of a light source is xcex and the numerical aperture of an object lens required to form the light spot is NA, the light spot diameter is effectively given by xcex/NA. If the light spot diameter used is reduced, the recording density can be increased, but the wavelength xcex of the light source is thought to be limited due to absorption by an optical element or the sensitivity characteristics of a detector, while increase of NA is effectively limited by the permitted amount of tilt of the medium. In other words, there is a limit to increase of recording density achievable by reduction of the light spot diameter.
As a means of overcoming this limitation, super-resolution media exist in the art which reduce the effective light spot diameter using the optical characteristics of the recording medium. Typical examples of this super-resolution medium technology are as follows:
(1) Magnetic super-resolution (Jpn. J. Appl. Phys 31 (1992), pp. 529-533)
(2) Super-resolution using inorganic oxide films (Joint MORIS/ISOM ""97 Post-Deadline Papers Technical Digest, pp.21-22)
(3) Super-resolution reading by mask layer using organic pigments (Preprint of the Soc. of Jpn. Appl. Phys. (1994-Fall), p.1000(19p-K-6))
(4) Super-resolution by photo-chromic mask layer (Optical Review 4 (1997), pp.655-659)
(5) Super-resolution reading using melting of phase change material (Jpn. J. Appl. Phys. 32 (1993), p.5210).
These techniques create an effect which masks recording marks so as to make the effective spot contributing to recording/read-out smaller by using the variation of temperature distribution or transmittance, and thereby increase the recording/read-out density.
FIG. 1 schematically shows this medium super-resolution effect. Alight spot 11 scans the surface of a super-resolution medium in a direction shown by 13, and thereby performs recording/read-out. During normal read-out, all recording marks 12 inside the light spot 11 contribute to the read-out signal, but in the case of super-resolution media, regions apart from a center region 14 in the light spot which has a strong light intensity are masked, and only a recording mark 15 inside the region 14 is read out. This is equivalent to reducing the effective light spot diameter contributing to read-out. Conversely to the example of FIG. 1, it is also possible to mask the region 14, and detect recording marks outside the region 14 with the light spot 11.
Method (1) can be applied only to magneto-optical disks, and cannot be applied to read only disks such as CD-ROM and DVD-ROM which are presently widely available. Methods (3) and (4) use organic materials as mask layers which are easily damaged by heat, consequently, the total number of possible reads is of the order of 10,000 or less, and as the reliability of reading out information is low, they have not been commercialized. Further as they are destroyed by heat, they cannot be applied to rewritable disks. Method (5) employs fusion of a phase change material in the super-resolution mask layer, so film flow occurs due to repeat reads, the number of possible reads is limited to about 10,000 or less, and as the reliability of information read-out is low, it also has not been commercialized. Moreover, as read-out is performed at a temperature above the melting point of the phase change material, with a rewritable disk, recording marks disappear at high temperatures during read-out, so this method can be used only for read-only disks. However, in Jpn. J. Appl. Phys. 38 (1999), p.1656, it is reported that by using a disk having an inorganic oxide super-resolution film, the disk can be read 100,000 or more times, and that a phase change medium applying this super-resolution film can be rewritten. This shows that, as method (2) employs inorganic materials, the disk,is not readily destroyed by heat as compared to a disk with organic materials. Due to this fact, inorganic oxide super-resolution films are expected to be applied as super-resolution materials suitable for both read-only disks and rewritable disks.
An inorganic super-resolution film has a property whereby its complex refractive index changes when it is irradiated by a laser beam of intensity exceeding a certain threshold. When this inorganic super-resolution film is applied to an optical disk, it will have a multi-layer structure such as shown in FIG. 2. When this optical disk is read out, the complex refractive index changes in the center part of the light spot where the temperature is high, and the reflectance also changes in the region where the complex refractive index changed due to multi-interference in the laminated film. As a result, the signal corresponding to part of the light spot is emphasized when the disk is read, and the effective spot diameter contributing to read-out is reduced.
A super-resolution film 23, substrate protecting film 22 and thermal buffer film 24 are designed with regard to the following three-stage mechanism.
(1) Light absorption occurs in the super-resolution film 23.
(2) The heat produced by-this light absorption is not dissipated by the reflecting film 25, but causes a temperature rise in the super-resolution film.
(3) The complex refractive index of the super-resolution film 23 changes, and the reflectance changes due to multi-interference in the laminated film.
The substrate protecting film 22 also has the role of preventing deformation of the substrate 21 due to the heat produced in the super-resolution film 23.
A prototype disk was designed and manufactured using an oxide film (referred to hereafter as a Coxe2x80x94Sixe2x80x94Naxe2x80x94Caxe2x80x94O film) comprising Co, Si, Na, Ca for the super-resolution film 23, a ZnSxe2x80x94SiO2 film for the substrate protecting film 22 and thermal buffer film 24, and an Alxe2x80x94Ti film for the reflecting film 25. The reflectance R (before change) and R (after change) was calculated relative to the film thickness of the substrate protective film 22 and thermal buffer film 24 taking account of multi-interference in the multi-layer film, assuming that the change of complex refractive index of the Coxe2x80x94Sixe2x80x94Naxe2x80x94Caxe2x80x94O film was from n (refractive index)=2.48 and k (extinction coefficient)=0.48, to n=2.41, k=0.57 when the incident light became intense. FIG. 3 is a plot of the calculated results for the reflectance variation rate (R (after change)xe2x80x94R (before change))/R (before change) assuming the film thickness of the Coxe2x80x94Sixe2x80x94Naxe2x80x94Caxe2x80x94O film was 50 nm and the film thickness of the reflecting film 25 was 100 nm. From these results, it is seen that the reflectance changes most when the substrate protective film 22 has a thickness in the range from 120 nm to 150 nm, and the thermal buffer film 24 has a thickness in the range from 30 nm to 50 nm. A disk prototype was therefore manufactured comprising a laminate of the substrate protecting film 22 (ZnSxe2x80x94SiO2 film) of thickness 120 nm, the super-resolution film 23 (Coxe2x80x94Sixe2x80x94Naxe2x80x94Caxe2x80x94O film) of thickness 50 nm, the thermal buffer film 24 (ZnSxe2x80x94SiO2 film) of thickness 30 nm, and the reflecting film 25 (Alxe2x80x94Ti film) of thickness 100 nm. The results of measuring reflected light intensity relative to incident light intensity on this prototype disk are shown in FIG. 4. In the case of result 41 where there is no super-resolution film (xe2x96xa1(empty squares)), the incident light intensity is directly proportional to the reflected light intensity, i.e., the reflectance does not vary even if the incident light intensity is increased. In the case of result 42 when the super-resolution effect is present (♦(black diamonds)), the direct proportional relationship breaks down when the light intensity is increased, and the reflectance drops. The results of measuring the amplified read-out signal from this disk are shown in FIG. 5. The recording marks are single frequency repeat phase pits (mark/space ratio=1:1), and the signal amplitude was normalized by the reflected light level. The phase pits were formed by physical imperfections on the substrate and were of such a depth that the phase difference was xcex/6. Measurements were performed with an optical disk tester at a wavelength of 660 nm and object lens NA of 0.6. Compared to the case of result 51 when the super-resolution effect was absent, result 52 when the effect was produced shows that the amplitude of a mark length of 0.28 xcexcm in the vicinity of the optical cut off (mark length 0.275 xcexcm) increases. However, above a mark length of 0.35 xcexcm, the amplitude again decreases and recording density increase characteristics cannot be obtained.
To investigate the reason for these characteristics, the effective read-out spot shape on the disk was calculated by a simulation. FIG. 6 shows results of plotting the ordinary read-out spot shape and the effective read-out spot shape in the spot scanning direction when the super-resolution effect was produced after normalizing by the respective peak values. Compared to an ordinary read-out spot 61, an effective read-out spot 62 when the super-resolution effect was present has an intensity distribution peak with a sharper rise but a broader width. As the width is broader, the overall resolution decreases, and the amplitude in the vicinity of the mark length of 0.35 xcexcm falls considerably. On the other hand, as the peak rises more sharply, the amplitude of the mark length of 0.28 xcexcm which is a high frequency component increases. This is considered to give rise to the read-out signal amplitude characteristics shown in FIG. 5.
It was clear that, in a construction where the reflectance falls due to change of complex refractive index of the super-resolution film, as the effective read-out spot diameter is larger, read-out signal amplitude characteristics which allow higher recording densities cannot be obtained.
It is therefore an object of this invention, which was conceived in view of the above problem, to provide an optical disk medium capable of high recording densities.
In view of the above object, the optical disk medium of this invention comprises a substrate and a laminated film comprising two or more layers of thin film, these layers being comprised of an inorganic material excepting for a recording layer of the thin film and a resin film protecting the thin film, wherein the layers of the thin film are in the solid-state when an information reproducing light is incident, and the reflectance of the thin film increases with increase of incident light intensity.
The thin film of the optical disk medium further comprises two or more groups on the substrate.
At least one of a disk identification signal, an optimum read-out light intensity value and an optimum recording light intensity value is recorded in an information recording/read-out region of the surface of the optical disk medium.
A trial read-out region is provided in the information recording/read-out part of the optical disk to set an optimum read-out light intensity value when the optical disk is read out.
A trial recording region is provided in the information recording/read-out part of the optical disk medium to set an optimum recording light intensity value or an optimum recording waveform pattern when information is recorded on the optical disk.
At least one layer of the laminated film of this optical disk medium is an oxide thin film comprising Co.
A thin film having optical constants different from those of the substrate is further interposed, directly or via another layer, between the oxide thin film comprising Co and the substrate.
The thin film having optical constants different from those of the substrate comprises Ge, Si and N.
Alternatively, the thin film having optical constants different from those of the substrate comprises Au, Ag, Al.
The reflectance during read-out from this optical disk medium is from 1.3 times to 1.7 times the reflectance at a light intensity approximately xc2xc that of the read-out light. Alternatively, the reflectance during read-out from this optical disk medium is from 3.0 times to 4.0 times the reflectance at a light intensity of approximately xc2xc the read-out light.